A Canadian bit-part actress bombarded Alec Baldwin with lovesick, demanding messages, packed her belongings and arrived uninvited at the actor's homes in Manhattan and the Hamptons, a US court has heard.

A Canadian bit-part actress bombarded Alec Baldwin with lovesick, demanding messages, packed her belongings and arrived uninvited at the actor's homes in Manhattan and the Hamptons, a US court has heard.

The case is expected to put the 30 Rock actor in an unsought role - a key witness against a woman who says she had a fling with him some years ago, which he denies.

Baldwin, 55, is expected to give evidence next week against actress Genevieve Sabourin, whose lawyer says she was merely trying to get answers after the actor lost interest in her.

But prosecutors say Baldwin was alarmed by a campaign of calls, text messages, emails and unexpected appearances by a woman he met twice over 10 years, in a creepy crusade that escalated as he got engaged to now-wife Hilaria Baldwin.

"There was nothing legitimate about these communications," Manhattan assistant district attorney Zachary Stendig said in an opening statement. A judge is hearing the case without a jury.

Sabourin, 41, from the Montreal suburb of Candiac, met Oscar-nominated Baldwin on the set of the 2002 sci-fi comedy The Adventures Of Pluto Nash, in which he had a cameo role and she was a publicist.

He told police that they had dinner together in 2010 and that their relationship was strictly professional.

But Sabourin harangued him with so many phone calls that he changed his number and she sent emails that ranged from "I want to be your wife now" to a bid to "conceive a mini-Baldwin", Mr Stendig said.

Although Baldwin asked her to leave him alone, she appeared in March 2012 in his driveway in Amagansett, New York, but left before police arrived, according to prosecutors.

A few days later, after news of Baldwin's betrothal broke, Sabourin turned up at a Manhattan movie screening he was hosting and security guards threw her out, according to the prosecution.

Then she arrived outside his Greenwich Village apartment building on April 8 2012, with her dog and a car loaded with her possessions, and was arrested on misdemeanour stalking and harassment charges.

But Sabourin and her lawyer Todd Spodek said her contacts with Baldwin were far less one-sided than prosecutors portrayed. "It was a communication, both ways," she said outside court.

She and Baldwin exchanged emails about matters as personal as her father's death and in 2010 he took her for a romantic dinner and accompanied her to a hotel for a sexual tryst, Mr Spodek said.

"You will hear about all of this attention given to Ms Sabourin throughout this time period, and then, suddenly, the cold shoulder," he told the judge. Her subsequent conduct was simply "looking for closure", not a crime, he said.

If convicted, Sabourin could face up to 90 days in jail on the top count alone.

Baldwin's publicist declined to comment on the case.

Baldwin and his wife married in July 2012. Their daughter, Carmen, was born in August.

The eldest of acting's four Baldwin brothers, Alec Baldwin's long list of credits range from a Tony-nominated performance as Stanley Kowalski in A Streetcar Named Desire on Broadway to playing action hero Jack Ryan in the 1990 film The Hunt for Red October.

He recently launched a talk show, Up Late with Alec Baldwin, on MSNBC.